The present invention relates to an electronic antenna decoupling device and process for suppressing a frequency modulated interfering signal, of a useful signal also modulated in frequency, received on a receiving antenna by coupling with a transmitting antenna situated in the immediate vicinity of the receiving antenna, the coupling occuring either directly or indirectly by multiple reflections from surrounding objects such as buildings.
The process and the device of the invention apply particularly to the case where the interfering signal, which is an attenuated and phase-shifted replica of the transmission signal, may be represented by an equation of the form: EQU c'(t)=kB cos (.omega..sub.1 t+.phi..sub.B (t)+.theta..sub.B +.lambda.) (1)
where:
B represents the amplitude of the transmission signal PA1 .omega..sub.1 represents the pulsation of the transmission carrier frequency PA1 .phi..sub.B (t) represents the phase or frequency modulation of the transmission signal PA1 .theta..sub.B the phase shift of the modulation signal PA1 k and .lambda. being respectively the attenuation and phase rotation coefficients introduced by the space separating the receiving antenna from the transmitting antenna. PA1 .epsilon..sub.k represents the slope of the straight line representing the attenuation in the neighborhood of frequency f.sub.o, EQU .omega..sub.1 =.omega..sub.o +.DELTA..omega., PA1 .lambda..sub.o represents the phase-shift of the filter at frequency f.sub.o, PA1 .epsilon..sub..lambda. represents the slope of the straight line representing the phase-shift in the neighborhood of frequency f.sub.o.
A device is known for suppressing at least partly the frequency modulated interfering signal received by an antenna. This device is formed by an assembly of elements which, from the interfering signal taken from the transmitting antenna, produce a compensation signal whose attenuation and phase rotation correspond to the attenuation and phase rotation which the interfering signal undergoes when passing from the transmitting antenna to the receiving antenna, on the assumption that the interfering signal undergoes, in the interantennae space, an attenuation and phase rotation which are constant in time, independent of the modulation signal .phi..sub.B (t). The compensation signal is subtracted from the composite signal received by the receiver so as to retain only the useful part of the signal received.
The adjustment of the attenuation k and phase rotation .lambda. values of the compensation signal is obtained by adjusting two variable attenuators whose adjustment values k.sub.1 and k.sub.2 are determined by the relationships EQU k.sub.1 =k cos .lambda. (2)
and EQU k.sub.2 =k sin .lambda. (3)
The device calculates first of all the differences EQU .epsilon..sub.1 =k cos .lambda.-k.sub.1 ( 4)
and EQU .epsilon..sub.2 =k sin .lambda.-k.sub.2 ( 5)
and a regulation loop adjusts the values k.sub.1 and k.sub.2 of the adjustable attenuators to cancel out these differences. At equilibrium, when the differences .epsilon..sub.1 and .epsilon..sub.2 are zero, the compensation signal has the same amplitude and phase characteristics as the interfering signal received.
Now, practice has shown that, when the regulating loop has reached its equilibrium, the signals .epsilon..sub.1 and .epsilon..sub.2 are not cancelled out and that there remains an AC residue for each of these signals which is phase coherent or in phase opposition with the modulating signal .phi..sub.B (t) of the transmitted interfering signal. It follows that the above-mentioned device does indeed cancel out the carrier frequency of the interfering signal but it remains inefficient for modulation frequencies. Thus, the components of the interfering signal are only partially eliminated.
The imperfect result thus obtained is partly explained by the fact that the attenuation and phase rotation undergone by the interfering signal during its travel in space between the antennae are not constants independent of the modulation signal .phi..sub.B (t). On the contrary, assumptions and tests carried out during elaboration of the device of the invention have shown that each frequency of the spectrum of the modulated wave undergoes attenuations and phase-shifts which depend on its position in the spectrum. It is apparent that in the proximity of the carrier frequency f.sub.o, the variations of the phase and of the amplitude of each frequency of the spectrum of the modulated wave received are practically linear as a function of its position in the spectrum. Consequently, it is possible to liken the transfer function of inter-antenna space to that of a linear filter of equation: EQU H.sub.(.omega.) =(k.sub.o +.epsilon..sub.k (.omega.-.omega..sub.1))e.sup.j(.lambda..sbsp.o.sup.-.epsilon..sbsp..lambd a..sup.)(.omega.-.omega..sbsp.1.sup.) ( 6)
where: k.sub.o is the attenuation coefficient of the filter at
frequency f.sub.o,
represents the pulsation of the carrier wave of the interfering signal and .omega..sub.o the pulsation of the carrier wave of the useful signal,
It follows that a frequency modulated interfering signal, able to be represented by the equation EQU c=B cos (.omega..sub.1 t+.phi..sub.B (t)+.theta..sub.B) (7)
is transformed after passing through the linear filter formed by the inter-antennae space into a signal EQU c'(t)=B/2(k.sub.o +.epsilon..sub.k .phi.'.sub.B (t-.epsilon..sub..lambda.)) cos (.omega..sub.1 t+.phi..sub.B (t)+.theta..sub.B (t)+.lambda..sub.o -.epsilon..sub..lambda. .phi.'.sub.B (t)) (8)
It follows that the coefficients k and .lambda. of equation (1) which were considered as constants in elaborating the prior device, depend in fact on time and may be represented by equations EQU k(t)=k.sub.o +.epsilon..sub.k .multidot..phi.'.sub.B (t-.epsilon..sub..lambda.) (9)
and EQU .lambda.(t)=.lambda..sub.o -.epsilon..sub..lambda. .multidot..phi.'.sub.B (t) (10)
where .phi.'.sub.B (t) represents the derivative of the first order of the phase modulation signal .phi..sub.B (t).